Amazon DynamoDB is an advanced, serverless key-value store that is fast and scales well, but that suffers from a complex API design. To learn more about DynamoDB, visit the Amazon DynamoDB webpage or buy the excellent "The DynamoDB Book" by Alex DeBrie for in-depth coverage with samples.
LambdaSharp.DynamoDB.Native uses LINQ expressions with a fluent interface to drastically simplify access to advanced DynamoDB features.
To optimize simplicity, the DynamoTable makes a few design choices.
DynamoTableis designed for single table access and does not support operations across multiple tables.DynamoTableassumes the primary key is made from a partition key (PK) and a sort key (SK).
These are intentional constraints of the DynamoTable implementation. If these constraints do not fit your use case, then DynamoTable may not be suitable for you.
The constructor for DynamoTable only requires the table name. Optionally, an IAmazonDynamoDB client can be passed as a second parameter. When omitted, the default AmazonDynamoDBClient is used.
IDynamoTable table = new DynamoTable(tableName);Once the DynamoTable instance has been initialized, it is ready for use.
Most DynamoDB operations use DynamoPrimaryKey to specify the item to act upon. The exception to this rule is the Query operation, which selects a collection of items.
The DynamoPrimaryKey specifies the partition key and sort key of the item to act upon. The primary key is always a composite key and the names of the partition key and sort key must be PK and SK, respectively.
var primaryKey = new DynamoPrimaryKey<CustomerRecord>(pkValue, skValue);The generic type parameter in DynamoPrimaryKey<TItem> serves two purposes:
- It avoids accidental confusion between primary keys for different items by having a stricter type.
- The generic type is used to determine the type of the item to instantiate.
The GetItemAsync() is the simplest way to retrieve a DynamoDB item. It reads all the item attributes and populates an instance. If the item was not found, the method returns null instead.
var customer = await table.GetItemAsync(primaryKey);The GetItem() method enables retrieving only some properties of the item instead of all of them.
The following code only populates the CustomerId and CustomerName properties.
var partialItem = await table.GetItem(primaryKey)
.Get(item => item.CustomerId)
.Get(item => item.CustomerName)
.ExecuteAsync();Reducing the amount of data returned does not reduce the DynamoDB read consumption, but it does reduce the bandwidth required to retrieve the wanted data.
The DynamoTable instance analyzes the lambda expressions passed into the Get() method and determines from them the DynamoDB attributes to request. The lambda expression is typed using the generic parameter from DynamoPrimaryKey<TItem>. The lambda expression can use . and index [] operations to specify inner attributes of maps and lists as well. This approach makes it both easy and safe to specify the wanted attributes.
A NotSupportedException is thrown when the lambda expression is not valid for specifying a DynamoDB attribute.
The PutItemAsync() method creates new DynamoDB item or replaces one. When an existing item is replaced, all previous item attributes are removed and replaced with the item attributes. The method returns a boolean indicating success.
var customer = new CustomerRecord {
CustomerId = "123",
CustomerName = "John Doe",
CustomerEmail = "john@example.org",
Addresses = new Dictionary<string, CustomerAddress>()
};
var success = table.PutItemAsync(primaryKey, customer)The PutItem() method allow specifying conditions and also returning the previously stored DynamoDB item.
Use the ExecuteReturnOldItemAsync() to retrieve the DynamoDB item attributes that were replaced. If the DynamoDB item doesn't exist, the method returns null.
var customer = new CustomerRecord {
CustomerId = "123",
CustomerName = "John Doe",
CustomerEmail = "john@example.org",
Addresses = new Dictionary<string, CustomerAddress>()
};
var previousCustomer = table.PutItem(primaryKey, customer)
.ExecuteReturnOldItemAsync()Use WithConditionItemDoesNotExist() to specify a condition before performing the PutItem operation. The following code checks that the DynamoDB item does not exist.
var customer = new CustomerRecord {
CustomerId = "123",
CustomerName = "John Doe",
CustomerEmail = "john@example.org",
Addresses = new Dictionary<string, CustomerAddress>()
};
var success = table.PutItem(primaryKey, customer)
.WithConditionItemDoesNotExist()
.ExecuteAsync()See Comparison Operator and Function Reference for a complete list of condition expressions.
Sometimes, it is necessary to store additional attributes alongside an item. For example, attributes used by either local or global secondary indices. This is possible by using the Set() method.
var customer = new CustomerRecord {
CustomerId = "123",
CustomerName = "John Doe",
CustomerEmail = "john@example.org",
Addresses = new Dictionary<string, CustomerAddress>()
};
var success = table.PutItem(primaryKey, customer)
// allow retrieving customer item using the email address via the global secondary index GSI1
.Set("GSI1PK", $"EMAIL={customer.CustomerEmail}")
.Set("GSI1SK", "INFO")
.ExecuteAsync()The DeleteItemAsync() method is used to delete a DynamoDB item. The operation is idempotent and succeeds regardless if the item exists or not.
var success = await table.DeleteItemAsync(primaryKey);The DeleteItem() method has similar capabilities as the PutItem() method and allows specifying a condition for the operation and returning the previously stored DynamoDB item.
Use the ExecuteReturnOldItemAsync() to retrieve the DynamoDB item attributes that were deleted. The method returns null if the DynamoDB item doesn't exist or the operation could not be performed.
var previousCustomer = table.DeleteItem(primaryKey)
.ExecuteReturnOldItemAsync()Use WithCondition() to specify a condition before performing the DeleteItem operation. The following code checks the state of the DynamoDB item before deleting it.
var success = table.DeleteItem(primaryKey)
.WithCondition(item => item.Status == CustomerStatus.Deactivated)
.ExecuteAsync()The DynamoCondition static class contains static methods representing native DynamoDB condition functions and operators. These methods are only used to analyze the lambda expressions in the WithCondition() method. When used directly, the methods throw an InvalidOperationException.
See Comparison Operator and Function Reference for a complete list of condition expressions.
The UpdateItem operation is the most powerful and versatile operation that DynamoDB supports. It allows updating specific attributes of a DynamoDB item without affecting others. Furthermore, it can use the previous value of the item and atomically modify it to a new value. Similar to the PutItem operation, the UpdateItem operation can either create a new DynamoDB item or update an existing one.
The UpdateItem() method allows tapping into the rich set of capabilities of the UpdateItem operation, which include:
- The
Set()method to modify a DynamoDB item attribute to a specific value or to compute a new value from an existing one. See SET—Modifying or Adding Item Attributes). - The
Remove()method to delete a top-level or nested attribute. See REMOVE—Deleting Attributes from an Item. - The
Add()method to add a number to a numeric attribute or add an item to a set attribute. See ADD—Updating Numbers and Sets. - The
Delete()method to remove an item from a set attribute. See DELETE—Removing Elements from a Set.
Use the ExecuteReturnOldItemAsync() to retrieve the DynamoDB item attributes before they were updated. The method returns null if the DynamoDB item doesn't exist or the operation could not be performed.
var previousCustomer = table.UpdateItem(primaryKey)
.Set(item => item.CustomerName, "John Doe")
.ExecuteReturnOldItemAsync()Use the ExecuteReturnNewItemAsync() to retrieve the DynamoDB item attributes after they were updated. The method returns null if the operation could not be performed.
var updatedCustomer = table.UpdateItem(primaryKey)
.Set(item => item.CustomerName, "John Doe")
.ExecuteReturnNewItemAsync()Use WithConditionItemExists() to specify a condition before performing the UpdateItem operation. The following code checks that the DynamoDB item exists.
var success = table.UpdateItem(primaryKey)
.WithConditionItemExists()
.Set(item => item.PendingOrders, item => item.PendingOrders + 1)
.ExecuteAsync()See Comparison Operator and Function Reference for a complete list of condition expressions.
Some of the Set() methods take a lambda expression as their second argument to compute a new value based on an existing DynamoDB item attribute. Although, this capability is extremely powerful, the kind of operations that can be performed is very limited. See SET—Modifying or Adding Item Attributes for a list of allowed operations and functions. The DynamoUpdate static class contains static methods representing native DynamoDB update functions. These methods are only used to analyze the lambda expressions in the Set() method. When used directly, the methods throw an InvalidOperationException.
Sometimes, it is necessary to store additional attributes alongside an item. For example, attributes used by either local or global secondary indices. This is possible by using the Set() method.
var success = table.UpdateItem(primaryKey)
.WithConditionItemExists()
// update customer email
.Set(item => item.CustomerEmail, "john@example.org")
// also update global secondary index GSI1 to allow retrieving customer item using the email address
.Set("GSI1PK", "EMAIL=john@example.org")
.ExecuteAsync()The Query operation allows finding multiple items in the main index or in a local/global secondary index.
The Query operation is anchored by the partition key (PK). Optionally, constraints on the sort key (SK) can be specified to narrow down the collection of DynamoDB items. However, the list of constraints is very limited.
SKis equal to some valueSKis less than some valueSKis less than or equal to some valueSKis greater than some valueSKis greater than or equal to some valueSKis between two bounds (inclusively)SKbegins with a sub-string
Those are all the possible constraints. They cannot be combined into more complex constraints either. This list is literally it. See Key Condition Expression.
In LambdaSharp.DynamoDB.Native, the combination of the PK value and the SK constraint is referred to as the query clause. It is one of the more complex concepts in DynamoDB to model, but the query clause seems to work well in practice.
The following operation finds all WebSocket connections belonging to a given user.
var clause = DynamoQuery.FromIndex("GSI1")
.SelectPK("USER=123")
.WhereSKBeginsWith("WS=");The Query() method can specify an operation that returns a list of DynamoDB items that all have the same type or they can be mixed. For the latter case, the WithTypeFilter<TItem>() method is used to specify what types to deserialize. Items returned by the DynamoDB Query operation that do not match the type filter are discarded.
The results of the DynamoDB Query operation can either be retrieved as a complete list using ExecuteAsync() or streamed incrementally using ExecuteAsyncEnumerable(). This example also shows how to use a cancellation token to cancel the operation.
Using ExecuteAsync():
// receive all items at once
var list = await table.Query(clause, limit: 100)
.ExecuteAsync(cancellationToken);Using ExecuteAsyncEnumerable():
// receive items as they come in
var asyncEnumerable = table.Query(clause, limit: 100)
.ExecuteAsyncEnumerable(cancellationToken);
await foreach(var item in asyncEnumerable.WithCancellation(cancellationToken)) {
// process item incrementally
}- Serialization for DynamoDB
- .NET Type Mappings for DynamoDB
- Custom Type Converters for DynamoDB
- AttributeValue to JSON converter
- Condition/Filter Expression Specification
- UpdateItem SET Expression Specification
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